UNAUDITED QUARTERLY DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
UNAUDITED QUARTERLY DATA
Revenues	$ 362,500	$ 399,746	$ 392,150	$ 330,909	$ 343,951	$ 384,870	$ 369,357	$ 313,388	$ 1,485,305	$ 1,411,566	$ 1,337,374
Gross profit (Revenues less cost of services provided)	180,265	204,257	201,941	162,866	168,972	196,060	186,715	152,080
Net Income	$ 31,749	$ 45,046	$ 45,073	$ 30,281	$ 29,917	$ 41,121	$ 40,860	$ 25,766	$ 152,149	$ 137,664	$ 123,330
Income per share:
Income per share Basic (in dollars per share)	$ 0.15	$ 0.21	$ 0.21	$ 0.14	$ 0.14	$ 0.19	$ 0.19	$ 0.12	$ 0.70	$ 0.63	$ 0.56
Income per share Diluted (in dollars per share)	$ 0.15	$ 0.21	$ 0.21	$ 0.14	$ 0.14	$ 0.19	$ 0.19	$ 0.12	$ 0.70	$ 0.63	$ 0.56